As early as 1912 beetles began showing signs of immunity to Paris green. Farmers didn’t notice, though, because the pesticide industry kept coming up with new arsenic compounds that kept killing potato beetles. By the 1940s growers on Long Island found they had to use ever-greater quantities of the newest variant, calcium arsenate. After World War II an entirely new type of pesticide came into wide use: DDT. Farmers bought DDT and exulted as insects vanished from their fields. The celebration lasted about seven years. The beetle adapted. Potato growers demanded new chemicals. The industry provided dieldrin. It lasted about three years. By the mid-1980s, a new pesticide in the eastern United States was good for about a single planting.

In what critics call the “toxic treadmill,” potato farmers now treat their crops a dozen or more times a season with an ever-changing cavalcade of deadly substances. Nonetheless, the pests keep coming back. Researchers were dismayed in the 1980s to discover that new types of P. infestans had found their way to Europe and America. They were more virulent—and more resistant to metalaxyl, the chief current anti-blight treatment. No good substitute has yet appeared.

In 2009, potato blight wiped out most of the tomatoes and potatoes on the East Coast of the United States. Driven by an unusually wet summer, it turned gardens into slime. It destroyed the few tomatoes in my New England garden that hadn’t been drowned by rain. Accurately or not, one of my farming neighbors blamed the attack on the Columbian Exchange. More specifically, he said blight had arrived on tomato seedlings sold in big-box stores. “Those tomatoes,” he said direly, “come from China.”